Semiconductor laser diodes are vital components of many communications, data storage and printing systems due to their compact size, performance and compatibility with electronic circuitry.
In order to improve device performance and reduce manufacturing costs, there has been considerable effort aimed towards opto-electronic device integration. An example of these efforts is the integration of laser diodes and monitor photodiodes. The photodiode, facing the back facet of the laser diode, is usually used for monitoring and stabilizing the laser output power. Specially designed photodiodes have also been used to monitor other laser beam characteristics such as the far field intensity distribution.
The following publications and patents are representative of the prior art for integrated laser diode/photodiode structures:
"Semiconductor Laser Having an Etched Mirror and a Narrow Stripe Width, With an Integrated Photodetector", to M. Kitamura et al (U.S. Pat. No. 4,470,143, September, 1984), describes an integrated laser diode and monitor photodiode which are formed by wet chemical etching of the facets. The facets are both vertical and the stripe width of the photodiode is wider than that of the laser diode in order to improve collection efficiency. The facet of the photodiode may be parallel or angled with respect to that of the laser diode or may have a cylindrical shape.
"GaAs:GaAlAs Ridge Waveguide Lasers and Their Monolithic Integration Using the Ion Beam Etching Process", by N. Bouadma et al (IEEE Journal of Quantum Electronics, vol 25, pp. 2219-2228, November, 1989), describes a laser diode integrated with a monitor photodiode. The vertical and sloped facets of the laser diode and photodiode, respectively, are etched using an ion beam process. The sloped facet of the monitor photodiode reduces the amount of light that is reinjected into the laser.
"An Integrated AlGaAs Two-Beam Laser Diode-Photodiode Array Fabricated with Reactive Ion Beam Etching", by M. Uchida et al (Electronics and Communications in Japan, part 2, vol 72, pp. 33-43, 1989), discusses the fabrication and evaluation of an independently addressable dual laser diode array with monolithically integrated monitor photodiodes associated with each laser diode. The facets of the laser diodes and photodiodes are vertical and parallel to each other and result from reactive ion etching a groove.
"Integrated Semiconductor Diode Laser and Photodiode Structure", to P. L. Buchmann et al (European Patent 0 410 067A1, January, 1991), discloses multiple monitor photodiode structures integrated with a laser diode, both devices with vertical facets. The various structures discussed, including those with a monitor photodiode at each laser facet, are used primarily for wafer scale testing. In particular, the facets of the monitor photodiodes are shaped to enhance their sensitivity to transverse mode changes in the laser diode.
"Full-Wafer Technology--A New Approach to Large-Scale Laser Fabrication and Integration", by P. Vettiger et al (IEEE Journal of Quantum Electronics, vol 27, pp. 1319-1330, June, 1991), describes wafer scale fabrication and testing of monolithically integrated laser diodes and monitor photodiodes. The device facets are formed by chemically assisted ion beam etching (CAIBE) with both the laser diode and photodiode facets perpendicular to the wafer surface. The facet of the monitor photodiode is placed at an unspecified angle with respect to the laser diode facet. In this work, the output facet of the laser diode received a low reflectivity coating while the back facet received a high reflectivity coating. The low reflectivity coating is also deposited on the light collection surface of the monitor photodiode.
As described above, in order to fabricated useful laser diodes, dielectric coatings are applied to both the output and back facets of the laser diode to adjust their respective reflectivities and, hence, the ratio of the powers emitted at each facet. Altering the facet reflectivities may also affect some performance parameters of the device such as the threshold current and differential quantum efficiency. An additional benefit of depositing dielectric coatings on the facets is that these films passivate the exposed semiconductor surface and, hence, improve device reliability. Depositing the dielectric coating to the monolithic laser diode/monitor photodiode structure results in the output facet coating material being deposited simultaneously on the light collection surface of the monitor photodetector. Due to variability in the fabrication process, the dielectric coating will exhibit variations in both thickness and composition. This variability will, in turn, lead to variations in the resulting facet relectivities and, hence, to variations in laser diode performance. In addition, the varying properties of the dielectric coating will also affect the monitor photodetector performance in altering the reflectivity of the light collection surface and, hence, its power detection efficiency.